Description

Structural transformation of lignin in pulps bleached with polyoxometalates (POMs) should be explored in order to gain insight into the lignin reactions leading to its solubilization during POM treatment of kraft pulp. Our studies include characterization of residual lignin isolated from softwood commercial and birch laboratory kraft pulps delignified with POMs. Oxidative treatment of unbleached kraft pulps, commercial mixed softwood and laboratory birch, was performed using complex POM solutions containing the active [SiVW11O40]5- anion. Kappa number and contents of Klason lignin and hexeneuronic acid groups (HexA) were determined in the pulps before and after POM treatment. It was found that during this treatment the HexA groups were removed from both pulps. The kappa number corrected for the HexA contribution (“HexA-corr kappa number”) was calculated for the pulps with the progress of POM delignification and the corresponding levels of POM reduction were measured. Based on these data, no difference in the delignification efficiency between softwood and birch kraft pulp was observed. The conversion factor between Klason lignin and “HexA-corr. kappa number,” which is used as a convenient method for estimating the lignin oxidizability level, gradually increased with the progress of POM delignification. This indicates that POM delignification reduces the level of oxidizable structures in lignin. Residual lignins of unbleached kraft and kraft POM delignified pulps were isolated by mild acid hydrolysis. Analyses of the residual lignins show that during POM delignification lignin undergoes changes that include a sharp reduction in the content of phenolic hydroxyl groups (PhOH groups) and a high increase in carbonyl groups, based on the “aromatic lignin.” Results strongly indicate that POM treatment of kraft pulps results in a loss of aromaticity. Stilbene structures characteristic of kraft pulp residual lignin are modified during POM delignification, and the stability of lignin bonds in this process decreases in the following order: â-O-4 > â-5 > â-â. Based on the results of this study it appears that in POM delignification the residual lignins of softwood and birch kraft pulps are removed in a similar manner. The different levels of carbonyl groups in the residual lignins isolated from pulps of approximately the same kappa number suggest, however, that the extent of the corresponding reactions may be different for guaiacyl (G), softwood and syringyl- guaiacyl (SG), birch lignins.

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